1. Field of the Invention
The present invention relates to a water-dispersion varnish for electrodeposition.
2. Description of the Prior Art
Heretofore, metal surfaces have been coated with an organic film which is electrically insulating, anti-corrosive or resistant to mechanical damage, by methods such as a tumbling coating method, an electrostatic coating method or an electrodeposition coating method. Among these coating methods, the electrodeposition coating method has many significant advantages: high efficiency of use of the varnish employed; shortening of the total time required for the process; ease of automation; and production of a coated film having no pin-holes. Moreover, the troublesome considerations of safety and hygiene are not a problem because no organic solvent is used. Accordingly, the electrodeposition coating method has been widely used for preparation of enameled wires.
The varnishes used for electrodeposition can be classified as: (1) a water-soluble type varnish for formation of a coated film prepared by dissolving compounds having molecular weights less than several thousands and having dissociating groups and (2) a water-dispersion varnish for formation of a coated film prepared by dispersing a polymer into water as fine particles. When the water-soluble type varnish is used, uniform deposition can be attained on the entire surface including that portion of the surface residing in a gap of a complicated shape. However, the thickness of the film formed by the electrodeposition is only several tens .mu.m. Accordingly, pin-holes are easily formed when using certain varnishes. Moreover, the electric insulating property of the coated film is quite low because of the dissociating groups introduced into the polymer in order to impart water solubility. That is, in the water-soluble type varnish, the compound employed has dissociating groups which dissolve as electrolytes in the water, whereby the coated film prepared from the varnish has inferior electric characteristics and is unsatisfactory as an electrical insulating film.
On the other hand, the water-dispersion varnish has a relatively poor covering performance. It is difficult to form a uniform film on a surface having a complicated shape. However, it has many offsetting advantages, namely: a thickly coated film can be deposited easily in a short time; the number of dissociating groups is small; and polymers having high molecular weights can be used so as to form a coated film having excellent mechanical properties, chemical resistance, water resistance, and electrical characteristics such as volume resistivity and breakdown voltage. Accordingly, in preparations of enameled wires by forming a coated film on a bare copper wire by the electrodeposition, the latter water-dispersion type varnish principally has been used. However, since the resultant enameled wire must simultaneously possess many different characteristics in the coated film such as uniformly good appearance with no swelling or cracking, excellent scrape abrasion resistance, high cut-through temperature, appropriate volume resistivity and mechanical characteristics and high chemical resistance, it has been very difficult to apply electrodeposition using a water-dispersion varnish. This occurs because it is difficult to select components, appropriate ratios of the components and polymerization conditions for the preparation of the water-dispersion varnish which can produce the desired result. Accordingly, a water-dispersion varnish suitable for preparing an enambled wire having all of the above-mentioned satisfactory characteristics has not been known. Consequently, it would be most desirable to have such a varnish which produces a coating possessing all these desirable properties simultaneously.